Holley 1920 linkage

The adjustment is at the vertical rod that runs up and connects to the bellcrank near the firewall, on the opposite leg of the bellcrank from the throttle return spring. To adjust it, you push the top of that vertical rod rearward towards the firewall until it snaps off its ball (the ball is on the bellcrank), then unscrew the top of the rod to make the linkage longer (which makes upshifts later and harder) or screw down the top of the rod to make the linkage shorter (which makes upshifts earlier and softer).

If your shifts are occurring at the right road speeds but the shift quality is poor, the problem is not the linkage.